The present specification relates to displays. More particularly, the present specification relates to head up displays (HUDs).
Conventional HUDs are generally large, expensive and difficult to fit into small airplanes. Often, conventional HUDs rely on large lenses to form adequate field of view and viewing eye box. Compact HUDs are needed for small business jets and other aircraft where space is constrained in the cockpit.
Substrate guided HUDs have been proposed which use waveguide technology with diffraction gratings to preserve eye box size while reducing lens size. U.S. Pat. No. 4,309,070 issued to St. Leger Searle and U.S. Pat. No. 4,711,512 issued to Upatnieks disclose substrate waveguide HUDS where the pupil of a collimating optical system is effectively expanded by the waveguide structure. However, such systems have faced difficulties in design. For example, U.S. Pat. No. 7,711,512 teaches the use of diffraction gratings based upon holographic materials which can be difficult to process consistently. Holograms are generally extremely angle-sensitive and wavelength-sensitive because they rely on low index modulation throughout a thick volume (Δn<0.05), where the required phase shift for diffraction can only be met for a small set of wavelengths and angles. Diffraction gratings fabricated using embossing and casting processes are more repeatable but can be limited to organic low refractive index materials, thereby limiting the field of view and spectral range. Diffraction gratings that have mechanically reproduced gratings (ruled gratings) often do not achieve the required resolution for HUD applications. Diffraction gratings implemented using a surface relief geometry provide favorable characteristics compared to the aforementioned types.
Substrate guided HUDs achieve a large eyebox size while reducing the collimating lens size by an affect called pupil expansion. Light from the collimating optics pupil is coupled into the waveguide substrate by a first surface relief diffraction grating. The pupil expansion effect comes about by using a relatively weak (low diffraction efficiency) second surface relief diffraction grating with a larger extent in the desired expansion direction compared to the first grating, so that light propagating within the waveguide interacts many times with the second diffraction grating, gradually coupling out along the length of the waveguide and effectively creating an expansion of the collimating optics pupil. The gradual out-coupling action means that at each subsequent interaction with the second diffraction grating there is less light available compared to the previous interaction, so the expanded pupil luminance becomes lower and lower along the length of the second diffraction grating. This luminance decrease is manifested as a luminance non-uniformity in the displayed HUD image. Therefore, there is a need for a means of balancing the expanded pupil luminance along the length of the surface relief out-coupling diffraction grating.